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Patent Issued for Method for Coating Micromechanical Parts with High Tribological Performances for Application in Mechanical Systems

July 23, 2014

By a News Reporter-Staff News Editor at Journal of Engineering -- The Swatch Group Research and Development Ltd (Marin, CH) has been issued patent number 8770827, according to news reporting originating out of Alexandria, Virginia, by VerticalNews editors.

The patent's inventors are Steinmuller, Detlef (Aldrans, AT); Steinmuller, Doris (Aldrans, AT); Drexel, Herwig (Igls, AT); Ghodbane, Slimane (Innsbruck, AT); Richard, David (Yverdon-les-Bains, CH); Cusin, Pierre (Villars-Burquin, CH).

This patent was filed on May 18, 2010 and was published online on July 8, 2014.

From the background information supplied by the inventors, news correspondents obtained the following quote: "The technical demands on micromechanical components are growing. Beside highest precision, there is a need to provide highest energy efficiency in the mechanical system, long lifetime and complete abdication of lubricants insofar as possible.

"In the last few years, a lot of documents dealing with this subject have been published. The described approaches fulfil some of the tasks without being able to deliver a complete solution mainly due to the restrictions on the materials used.

"Micromechanical components produced by mechanical machining (die cutting or shape cutting) exhibit two major disadvantages. First, they are either high priced or economically profitable only in mass production because investments are required for expensive production tools. Second, these processes reach their technical limits with an accuracy level lying by +/-5 micrometers.

"Thus several alternative approaches have been discussed in literature. One of the most promising idea relates to the etching of micromechanical parts from silicon wafers achieving highest precision, even exceeding the results of the machining techniques by far. The tolerances can be reduced to the sub-micrometer range, but at the expense of lifetime: in practical results it was shown that the mechanical strength as well as the abrasive wear of these parts could not meet the demands in the absence of lubricant. One solution to the problem has been demonstrated in EP patent No 1 904 901: by treating the surface of the micromechanical parts with oxygen the strength and the lifetime could be extended, but without achieving an ultimate solution however.

"The tribological performances could be enhanced by using special oils in mechanical systems, however at the expense of the demand for a dry running system.

"The longest lifetime is achieved by classical machined parts made of steel, but these systems have reached their limits in regard to high accuracy and have to be further lubricated.

"A further problem of the lubricated systems is the necessity for frequent service intervals, where the movements have to be cleaned and re-lubricated.

"Thus the cycles of operation are limited and additional costs arise. These service intervals are necessary due to aging of the used oils which are loosing their properties with time.

"Numerous approaches to fulfil all these requirements with one system have been undertaken.

"In EP0732635B1 an approach is described 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 therefore a subsequent polishing if the diamond coated parts are used in sliding contact applications.

"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.sup.2 content (ranging from 30-100%) and is amorphous carbon which hardness is not sufficient for effective wear protection applications.

"EP1622826 discloses a micromechanical component comprising a first surface and a second surface, which are substantially perpendicular in relation to one another wherein the first and/or the second surface consist at least partially of diamond.

"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.

"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 be means of plasma processes. The aim of this post-treatment is the degradation of the top layers of the sp.sup.3-hybridised diamond coating into sp.sup.2-hybridised carbon species. The expectation 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.sup.3 diamond on top of which is a top layer of several hundred nanometers of sp.sup.2 hybridised amorphous carbon. The top layer is relatively soft and will wear off quickly in applications involving high friction.

"All the solutions described above can solve only partly the problem of providing micromechanical components featuring coefficient of friction lower than 0.05, and thus a large-scale production was prevented which is however demanded in the watch industry for instance.

"In particular, when using diamond-coated silicon, the solutions described above raised the following problems: the diamond coated micromechanical components often exhibit a high initial coefficient of friction because of the microcrystalline structure of the diamond coatings. This high coefficient of friction severely limits the efficiency of the micromechanical system during the first hours of its life.

"It is well known that surfaces with a roughness above several hundred nanometers cannot achieve straightforwardly low coefficients of friction. Moreover, utilising coarse diamond films in mechanical systems require a very smooth counterpart. In such cases the rough diamond film would grind into its corresponding counterpart leading to a very quick abrasive wear of the system and its breakdown.

"In theory, a special case is imaginable, where different roughness modules are fitting special conditions and therefore produce a low coefficient of friction. However the pressure on each single grain would be too high leading to a breaking and/or interlocking of the grains. The mechanical system would thus lose its properties quickly ending up in a high coefficient of friction and thus a blocking of the system. After the breakdown of the coating, the whole system would collapse and/or damage the whole watch.

"Solutions suggesting a polishing after the diamond-coating and therewith a smoothening of the surface of the micromechanical components failed because of high costs, the low efficiency and an essential technical reason: the most important functional surfaces are the flanks of the micromechanical parts which are not accessible for mechanical polishing when mounted in a wafer. A polishing after removing the parts from the wafers is not easy and moreover uneconomic because of the multitude and the diminutiveness of the micromechanical parts. Solutions with plasma etching of a diamond coated wafer comprising the micromechanical parts have also failed due to non-homogeneities of the plasma polishing especially on the flanks of the parts which are the most important areas (see above).

"Approaches using smaller crystal sizes (few hundreds nanometers) suffered similar problems in smaller dimensions. For instance, plasma etching of the flanks is not feasible because this process affects mainly the grain boundaries and etches the surfaces in an anisotropic way.

"Additionally, anisotropy of the etching treatment can arise from several parameters. The etching efficiency depends strongly on crystallographic orientations of the diamond crystals. As diamond films grown on substrates other than diamond (silicon in most cases) exhibit a mixture of crystallographic orientations, then etching is non-uniform, which can even increase the surface roughness of diamond instead of decreasing it."

Supplementing the background information on this patent, VerticalNews reporters also obtained the inventors' summary information for this patent: "A general aim of the invention is therefore to provide a process allowing providing micromechanical components for a micromechanical system, in particular a watch movement, providing a longer lifetime with high tribological performances.

"A further aim of the invention is to provide a process allowing providing micromechanical components for a micromechanical system, in particular a watch movement, reducing service intervals.

"Still another aim of the invention is to provide micromechanical components for a micromechanical system, in particular a watch movement, enhancing tribological performances, reducing wear and friction among others.

"These aims are achieved thanks to a process of 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.

"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 diamond coating is provided by CVD (chemical vapour deposition) in a reaction chamber; during CVD deposition, during the last portion of the growth process, a controlled change of the carbon content within the reaction chamber is provided, thereby providing a change of the sp.sup.2/sp.sup.3 carbon bonds in the vicinity of the surface.

"Thanks to the sp.sup.2-hybridised components, a lubrication effect is accomplished leading to a further decrease in the coefficient of friction. In parallel, the reduction of the surface roughness also leads to an enhancement of the tribological performances. It is important to note that a single surface layer of sp.sup.2-hybridised carbon on top of the diamond film or a diamond-like carbon film (DLC) would not deliver the same result since such a layer or film would wear fast due to the lower hardness and thus might be removed within a few cycles of the mechanical system. Moreover the adhesion of the sp.sup.2-hybridised layer to the sp.sup.3-hybridised matrix is far better when changing the sp.sup.2-hybridised carbon contents gradually, as shown in FIG. 3.

"In an advantageous embodiment, the change is an increase of carbon content in the diamond layer. This increase is obtained by increasing the proportion of carbon containing reactant gas, such as for instance methane.

"In a further embodiment, the increase of carbon content is obtained by adding an additional carbon containing gas, such as for instance acetylene.

"In a still further embodiment, the increase of carbon content is obtained by increasing the temperature and/or pressure within the processing reaction chamber.

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

"In a further advantageous embodiment, the increase of carbon content is obtained by replacing hydrogen (by up to 100% of the hydrogen content) by argon (or other element such as for instance nitrogen).

"In a still further advantageous embodiment, the increase of carbon content is obtained by adding at least one of the rare gases (namely neon, helium, krypton or xenon).

"In a variant, the increase of carbon content is obtained by lattice distortion.

"In another variant, the increase of carbon content is obtained by post-treatment, such as for instance plasma or laser treatment.

"In a variant, post-treatment consists in termination of the diamond-surface, wherein said termination is provided using an element selected from the list consisting of hydrogen, halogens, metals, electrical conductive minerals/organic molecules or proteins.

"In another variant, post-treatment consists in adding metal containing compounds on top of the diamond in order to decrease the sticking properties.

"In an advantageous variant, after the CVD steps, grain size is reduced (this enables to reduce surface roughness).

"The invention further provides a micromechanical component for a micromechanical system, in particular a watch movement, obtained by a method as previously described. In an advantageous embodiment, the surface layer has a gradual increase of sp.sup.2 hybridised carbon content.

"Such mechanical components, produced for micromechanical systems allow the running of the mechanical system under dry (unlubricated) conditions, feature high tribological performances (very low friction coefficient, reduced wear, etc), are long-time stable and feature high energy efficiency."

For the URL and additional information on this patent, see: Steinmuller, Detlef; Steinmuller, Doris; Drexel, Herwig; Ghodbane, Slimane; Richard, David; Cusin, Pierre. Method for Coating Micromechanical Parts with High Tribological Performances for Application in Mechanical Systems. U.S. Patent Number 8770827, filed May 18, 2010, and published online on July 8, 2014. Patent URL:

Keywords for this news article include: Silicon, Nanotechnology, Plasma Etching, Emerging Technologies, The Swatch Group Research and Development Ltd.

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Source: Journal of Engineering

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