Black decorative element for watchmaking or jewelry

Abstract

Decorative element (100) for watchmaking or jewelry, said decorative element (100) being provided with a black optical coating (102) and comprising: - a substrate (1); - a multilayer coating (200) provided on said substrate (1), wherein said multilayer coating (200) comprises at least two sequences of sublayers (2, 3) each comprising at least one light-absorbing sublayer (2) and at least one transparent sublayer (3), said multilayer coating (200) being configured so as to give said decorative element (100) a substantially black appearance. According a first embodiment of the invention, said substrate (1) is monocrystalline silicon and has an Ra roughness of between 1.5 µm and 10 µm, the roughness being formed by a substantially random pyramidal surface texture with a base angle of 54°- 55°. According to a second embodiment of the invention, said substrate (1) has an Ra roughness of between 0.02 µm and 4 µm, preferably between 0.05 μm and 1 μm, and in that said roughness is formed by a pyramidal surface texture with a base angle of 35° to 70° preferably 45° to 60°.

Description

DescriptionBLACK DECORATIVE ELEMENT FOR WATCHMAKING OR JEWELRYTechnical Field

[0001] The present invention relates to the field of watchmaking and jewelry. More particularly, it relates to a visible decorative element for watchmaking or jewelry which is of a substantially black colour.State of the Art

[0002] In the field of watchmaking or jewelry, particular difficulty is encountered when manufacturing deeply black-coloured elements such as dials, hands, case elements, and visible elements of pieces of jewelry.

[0003] Traditionally, black materials such as obsidian were used, as well as substrates coated with lacquers containing carbon black, both of which are particularly susceptible to specular reflection. More recently, carbon nanotubes have been used in such lacquers as a replacement for carbon black, which represents an improvement in the black appearance, but still suffers from the issue of specular reflection from the lacquer surface. Furthermore, such lacquers are not hard-wearing and are hence limited to visible elements which are protected by a glass or crystal and hence cannot be touched by the wearer. An example of a watch dial provided with a carbon nanotube lacquer coating, or carbon nanotubes encapsulated in a resin, is disclosed in US2018157214.

[0004] Specular reflection can be reduced, or even substantially eliminated, by eliminating the use of lacquer, and directly depositing carbon nanotubes on the surface to be rendered black, an example of such a treatment being Vantablack®. In these processes, carbon nanotubes are vapour deposited so as to be directly grown on a substrate. This gives a reflectivity of under 1 %, but is extremely fragile, and the surface cannot tolerate even the slightest touch with a tool, which presents handling difficulties and ensures that such coatings can only be used on surfaces protected by a glass or a crystal.

[0005] US2024 / 111251 describes a watch component, which may be a dial, case, dial ring, glass edge, movement, handle or oscillating weight, provided with a blackmultilayer coating comprising alternating light absorbing layers (e.g. metal layers) and wavelength correction layers (e.g. of Ta2Os , SiO2, TiO2, AI2O3 , ZrO2, Nb2Os, HfO2, Na5AI3Fi4, Na3AIF6, AIF3, MgF2, CaF2, BaF2, YF3, LaF3, CeF3, or NdF3). The substrate is textured with a regular pattern of recesses, with oblique surfaces where the ratio of the height to the width is greater than 1 :2, and the depth of the recesses is several tens of microns. Taking the lower bound of “several tens of microns” to be 20 pm, this gives an Ra value of 5 pm, where Ra is the arithmetic average of profile height deviations from the mean line. However, the level of blackness achieved by this arrangement of layers can be improved upon, further reducing the reflectivity.

[0006] The aim of the present invention is hence to at least partially overcome at least some of the above-mentioned drawbacks of the prior art.Disclosure of the Invention

[0007] More precisely, in a first embodiment the invention relates to a decorative element for watchmaking or jewelry provided with a black optical coating, as defined in claim 1. This decorative element comprises a substrate of monocrystalline silicon, upon which is provided a multilayer coating.

[0008] Said multilayer coating comprises at least two sequences each comprising at least one light-absorbing sublayer and at least one transparent sublayer, said multilayer coating being configured so as to give said decorative element a substantially black appearance when the multilayer coating is viewed. It should be noted that further sublayers may be interdisposed between the sublayers mentioned, and / or between the sublayers mentioned and the substrate, and it does not matter whether the first of the mentioned sublayers is a lightabsorbing sublayer or a transparent sublayer. The various light-absorbing sublayers may be of the same material, or different materials, and this applies equally to the various transparent sublayers. Various parameters of the sublayers to achieve a substantially black appearance are known in the art as such and can be generated by known thin film optical simulation models, and such a large range of solutions are possible such that it is not necessary to define them here, although preferred parameters are disclosed below.

[0009] According to the invention substrate has an Ra roughness of between 1 .5 pm and 10 pm, and said roughness is formed by a substantially random pyramidal surface texture with a base angle of 54°-55°, considered as per convention with respect to the plane of the substrate. This base angle is nominally typically substantially 54.7°. This surface topography is followed by each sublayer of the multilayer coating, such that each sublayer also has substantially the same roughness.

[0010] This range of Ra roughness of the surface of the substrate, in combination with the number of sequences of sublayers, has been shown to have an unexpected and surprising technical effect of giving a particularly deep black appearance with weak angular dependence, by limiting specular reflection above and beyond that attainable with lacquers, paints and dyes. Furthermore, by using a layer stack of light absorbing and transparent sublayers, the fragility of the coating is significantly reduced compared to lacquers, paints, direct- deposited carbon black or carbon nanotubes, or similar, particularly when the outermost sublayer is a hard transparent dielectric layer. As a result, the coating is less subject to soiling or damage during handling, and can even be used unprotected or cleaned in certain cases. Also, since the resulting multilayer coating substantially does not transmit light, the optical properties of the substrate are not important, such that any suitable material can be used without influencing the optical properties of the multilayer coating. Furthermore, the surface texture is easily achieved by alkaline etching of a monocrystalline silicon wafer, and results in particularly advantageous angles of internal reflection for capturing light within the multilayer coating which follows the same topography and texture.

[0011] In another embodiment, as defined in claim 2, the decorative element comprises a substrate of any convenient material, upon which is provided a multilayer coating.

[0012] Said multilayer coating comprises at least two sequences each comprising at least one light-absorbing sublayer and at least one transparent sublayer, said multilayer coating being configured so as to give said decorative element a substantially black appearance when the multilayer coating is viewed. It should be noted that further sublayers may be interdisposed between the sublayersmentioned, and / or between the sublayers mentioned and the substrate, and it does not matter whether the first of the mentioned sublayers is a lightabsorbing sublayer or a transparent sublayer. The various light-absorbing sublayers may be of the same material, or different materials, and this applies equally to the various transparent sublayers. Various parameters of the sublayers to achieve a substantially black appearance are known in the art as such and can be generated by known thin film optical simulation models, and such a large range of solutions are possible such that it is not necessary to define them here, although preferred parameters are disclosed below.

[0013] According to the second embodiment of the invention, the substrate has an Ra roughness of between 0.02 pm and 4 pm, preferably between 0.05 pm and 1 pm, and said roughness is formed by a regular or substantially random pyramidal surface texture with a base angle of 35°-70°, preferably 45° to 60°. This gives the same advantages as stated above in respect of the first embodiment, and it has been surprisingly discovered that the smaller Ra range gives greater flexibility in the base angle of the pyramidal surface texture, and hence the material of the substrate, which is hence not limited to monocrystalline silicon, although this is a preferred material.

[0014] Advantageously, said at least one light-absorbing sublayer has an extinction coefficient above 0.1 at least for wavelengths between 400 nm and 800 nm, and may advantageously be a metal, a metal alloy, a light-absorbing nitride, or a light-absorbing carbide alloy. These can be deposited e.g. with standard vapour deposition processes as appropriate.

[0015] Preferably, the multilayer coating does not contain any chromium, nor a chromium alloy, nor or a chromium compound.

[0016] Advantageously, said at least one transparent sublayer has a refractive index between 1 .3 and 2.5 at least for wavelengths in the 400 nm to 800 nm range, and may be a transparent dielectric material, such as a transparent oxide (such as silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, niobium oxide or similar) or transparent nitride (such as silicon nitride, aluminium nitride or similar). These can again be deposited, e.g. with standard vapour deposition processes as appropriate, and are very hard and hence wear-resistant.

[0017] Advantageously, said substrate is glass, metal, ceramic or silicon (of any crystallinity, particularly monocrystalline silicon), although glass-ceramic, alumina, silicon oxide, silicon nitride, silicon carbide are also possible.

[0018] Advantageously, said substrate is monocrystalline silicon, and wherein said roughness is formed by a substantially random pyramidal surface texture. This is easily achieved by alkaline etching of a monocrystalline silicon wafer, and results in particularly advantageous angles of internal reflection for capturing light within the multilayer coating which follows the same topography and texture.

[0019] Advantageously, said multilayer coating has a thickness between 150 nm and 1000 nm, preferably between 200 nm and 600 nm.

[0020] Advantageously, said multilayer coating comprises from 2 to 6 of said sequences, preferably 2 or 3 of said sequences. Typically, a total of at least 5 sublayers is present in the multilayer coating.

[0021] Advantageously, at least some, preferably all, of said least one light-absorbing sublayer and said at least one transparent sublayer have a thickness between 1 nm and 500 nm, preferably between 10 nm and 200 nm.

[0022] The decorative element may be, for instance, a watch dial, a watch hand, a bezel, an element of a watchcase, a visible element of a piece of jewelry or similar.Brief Description of the Drawings

[0023] Further details of the invention will become more apparent upon reading the following description, with reference to the annexed figures in which:- Figure 1 is a schematic cross-sectional view of a timepiece element or jewelry element provided with a black optical coating according to either embodiment of the invention;- Figure 2 is a photograph of a sample produced according to the first embodiment of invention;- Figure 3 is a graph illustrating the reflectivity performance of a sample produced according to the second embodiment of the invention;- Figure 4 is a 3D rendering from an atomic force microscope view of a sample produced according to the second embodiment of the invention;- Figure 5 is a selection of four profiles taken on the image of figure 4;- Figures 6-8 scanning electron micrographs of a cross section and surface of an anisotropically-etched monocrystalline silicon substrate.- Figures 9-10 examples of atomic force microscope anisotropically- etched monocrystalline silicon substrate with smaller Ra.- Figure 11 is a photograph of a sample produced according to the second embodiment of invention.Embodinents of the Invention

[0024] Figure 1 illustrates a timepiece element or jewelry element 300 provided with a black optical coating 100 according to the invention.

[0025] Optical coating 100 comprises a substrate 1 , which in the first embodiment is monocrystalline silicon, but in the second embodiment is typically of glass, metal, ceramic, glass-ceramic, alumina, silicon oxide, silicon nitride, silicon cabride or silicon itself (with any crystallinity), although other materials are possible, upon which a multilayer coating 200 is deposited. Substrate 1 may be mounted on a support 4, which is part of a timepiece or piece of jewelry (e.g. a watch hand, a bezel, a dial, a watch case component, or a support 4 of a piece of jewelry). Alternatively, the substrate 1 may itself be part of a timepiece or a piece of jewelry.

[0026] Multilayer coating 200 comprises at least two sequences of sublayers, preferably from 2 to 6 of said sequences, further preferably 2 or 3 sequences thereof. Typically, the total number of sublayers comprised by the multilayer coating is 5 or more, preferably between 5 and 13.

[0027] Each sequence of sublayers comprises a light-absorbing sublayer 2 and a transparent sublayer 3, said multilayer coating 200 being configured so as to give said decorative element a substantially black appearance, for which specific parameters will be discussed below. In terms of materials, the lightabsorbing sublayers 2 typically have an extinction coefficient above 0.1 at least for wavelengths in the 400 to 800 nm range, if not in a broader wavelengthband. Furthermore, the transparent sublayers 3 typically have a refractive index in the 1 .3 to 2.5 range at least for wavelengths in the 400 to 800 nm range, and are typically a dielectric material or materials.

[0028] The light-absorbing sublayers 2 may be all of the same material or different materials, with the same or different thicknesses, and are typically chosen from the following options:• metals, such as Al, W, Ni, Cr, Ag, Cu, Zn or similar, or their alloys,• light-absorbing nitrides, such as TiN, CrN or similar,• carbide alloys, such as WC, DLC (diamond-like carbon) or similar.

[0029] The transparent sublayers 3 may again be all of the same material or different materials, with the same or different thicknesses, and are typically chosen from the following options:• Transparent oxides, such as silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, niobium oxide or similar,• Transparent nitrides, such as silicon nitride, aluminium nitride or similar.• T ransparent oxynitrides such as silicon oxynitride or similar.• T ransparent polymers and / or resins.

[0030] The total thickness of the multilayer coating 200 is typically between 150 nm and 1000 nm, preferably between 200 nm and 600 nm, though thinner or thicker multilayer coatings 200 are possible. Each individual sublayer 2, 3 typically has a thickness between 1 nm and 500 nm, preferably between 10 nm and 200 nm, although thinner or thicker sublayers are also possible. Although chromium is mentioned as an option above in the multilayer coating 200, this is preferably absent in both the light-absorbing sublayers 2 and in the transparent sublayers 3 in all of its forms, i.e. as both elemental chromium, chromium alloys and chromium compounds.

[0031] Deposition of each sublayer in the multilayer coating 200 can be carried out with any convenient process for the materials chosen, such as for instance PVD (e.g. sputtering), CVD, ALD, LPCVD or PECVD for dry processes, which are preferred, or spray coating, spin coating, printing, electroplating, electroless (autocatalytic) plating or sol-gel processes for wet processes.

[0032] It should further be noted that the presence of other sublayers interposed within the sublayer stack as illustrated is possible, as is the presence of asupplemental layer, such as an adhesion layer, between the substrate 1 and the first light-absorbing sublayer 2 or transparent sublayer 3, as may be the case. It should also be noted that, although the first sublayer provided upon the substrate 1 is illustrated as being a light-absorbing sublayer 2, this could also be a transparent sublayer 3, provided that the multilayer coating comprises at least two sequences of a light-absorbing sublayer 2 and a transparent sublayer 3, considered either from the substrate 1 towards the free surface or inversely. In principle, the outermost layer is usually transparent, but a light-absorbing layer is also possible.

[0033] Furthermore, since the effect provided by the multilayer coating 200 is based on interference effects, multiple solutions are possible and the design of the multilayer coating 200 can be optimised with a thin film optical simulation program. Typically, between 5 and 13 sublayers total is optimal.

[0034] According to a first embodiment of the invention, the substrate 1 is monocrystalline silicon and has an Ra roughness of between 1.5 pm and 10 pm, which is achieved by chemical treatment (e.g. anisotropic wet etch), The base angle of the substantially random pyramids thereby produced is between 54° and 55°, typically substantially 54.7°, considered with respect to the plane of the substrate. Due to the thickness of the deposited sublayers in the multilayer coating 200, this roughness is followed by the sublayers 2, 3 of the multilayer coating 200, which hence also have substantially the same roughness and surface topology.

[0035] According to a second embodiment of the invention, the substrate 1 has a pyramidal Ra roughness of between 0.02 pm and 4 pm, preferably between 0.05 pm and 1 pm which can be achieved by chemical treatment (e.g. anisotropic wet etching), with a base angle between 35° and 70°, preferably between 45° and 60°. Another possibility is to apply a textured layer, such as a textured polymer layer, to a support, such that the support and the textured layer together form the substrate 1 . Due to the thickness of the deposited sublayers in the multilayer coating 200, this roughness is followed by the sublayers 2, 3 of the multilayer coating 200, which hence also have substantially the same roughness and surface topology

[0036] A particularly advantageous method for texturing the substrate 1 , also in the context of the second embodiment, is alkaline texturing of a monocrystalline silicon substrate 1 , which is known to create a random pyramid surface texture with a nominal base angle of between 54° and 55°, preferably substantially 54.7° (although in practice greater variation is observed), which, for normal light incidence, and considering the 2D case, results in reflection in one pyramid facet followed by 18° one on another facet. If the angle of incidence deviates from the normal, it can be shown that there is at least one reflection with incidence lower than 36°, trapping light inside the multilayer coating 200. In the 3D case of random silicon texturing the situation is a somewhat more complex but the effect is similar. For all incident angles, a low incidence reflection is obtained, and hence, thus we reduce the effect of increased reflectivity at high angles.

[0037] As a result of the stated roughnesses, reflectivity below 1 % can be easily achievable, which is difficult to attain with conventional paints and dyes or pigments. Furthermore, the extremely low reflectivity has weak angular dependence, and eliminates the typical glossy effect of usual black coatings, which have specular reflectivity which approaches 100% when light incidence is normal to the surface. This is achieved not only for the higher Ra values of the first embodiment with the narrow base angle range, but also for the smaller Ra values of the second embodiment with the broader base angle range, this latter embodiment also permitting a wider range of suitable materials for the substrate, not just monocrystalline silicon.

[0038] In addition, the surface hardness is significantly improved over lacquers, paints, direct-deposited carbon black or carbon nanotubes, or similar, particularly when the outermost sublayer is a hard dielectric layer such as an oxide or nitride as discussed above. As a result, the coating 200 is less subject to soiling or damage during handling, and can even be used unprotected or cleaned.

[0039] Also, the multilayer coating 200 has essentially zero transmission, so can be used on any convenient substrate 1 , irrespective of its optical properties, since these have substantially no impact on the end result.

[0040] Finally, the processing of the substrate 1 and the deposition of the sublayers 2, 3 of the multilayer coating 200 can be carried out using widely-available standard equipment using well-understood, standard processes.

[0041] An experiment was carried out on a sample according to the first embodiment, with the following structure:

[0042] As can be seen, the first sublayer provided on the substrate 1 is a light absorbing sublayer 2, and towards the free surface of the multilayer coating 200, two different transparent sublayers 3, namely of Nb20s and SiO2, are provided adjacent to one another, and 5 sequences each comprising at least one a light-absorbing sublayer and at least one transparent sublayer are provided.

[0043] This was observed to give a deep and homogeneous black colour, as illustrated in figure 2.

[0044] A study was also carried out in respect of a sample according to the second embodiment, with the following structure, with an Ra value of 0.8 pm:

[0045] As can be seen from the graph of Figure 3, which illustrates measurements of a sample fabricated according to the table immediately above, the reflectivity is extremely low over the whole visible range of 380 nm to 780 nm, and that this is attributable not only to the layer stack as such but also to the substrate texture, the difference between the two curves being simply whether the substrate is flat or textured in accordance with the invention and the difference being clearly apparent. The measured values were in good agreement with simulated results.

[0046] A photograph of a produced sample according to the second embodiment is shown in Figure 11.

[0047] Figure 4 illustrates a 3D rendering obtained from atomic force microscopy image of a sample produced as described immediately above, illustrating the random pyramidal structure of the anisotropically-etched monocrystalline silicon substrate 1.

[0048] In terms of roughness, the pyramid size and thus the roughness can be controlled by the process parameters. For 1 D profiles taken along a line of the surface, these are characterized by line roughness is Raand considering a whole 2D surface, the parameter is Sa. These are defined below:

[0049] Ra. Arithmetic average of profile height deviations from the mean line. Or mathematically:

[0050] Where lris the profile total length, and z(%) is the vertical deviation from the mean line as a function of position x along the line.

[0051] Sa. Arithmetic average of height deviations from the mean plane. Or mathematically:1 CCS“=S lz(x,y) dxdy s

[0052] Where S is the total surface, and z(%, y) is the vertical deviation from the mean line as a function of position x,y along the plane.

[0053] In the case of the pyramidal texture discussed above, Ravalues averaged over a large number of line profiles and Saroughness are close enough in practice as to be essentially interchangeable metrics, and can hence be used interchangeably, i.e. for the first embodiment, Sais between 1.5 pm and 10 pm, whereas for the second embodiment, Sais between 0.02 pm and 4 pm, preferably between 0.05 pm and 1 pm,. To illustrate this, a sample produced by the same method as that used for Figure 4, measured by confocal microscopy, resulted in an Raroughness averaged over 37 line profiles of 0.766 pm, whereas Saover the measured plane was 0.783 pm.

[0054] Figures 6-8 illustrate scanning electron micrographs of a cross section and the surface of a substrate prepared by anisotropic etching of a monocrystalline silicon substrate 1 , illustrating the pyramidal structure.

[0055] Finally Figures 9 and 10 illustrate atomic force microscopy images (top, plan views) of two other textured samples obtained according to the second embodiment of the invention, the difference being slight variations of the etching solution leading to Ra values of approximately 0.3 pm (figure 9) and approximately 0.08 pm (figure 10).

[0056] Although the invention has been described in terms of specific embodiments, variations thereto are possible without departing from the scope of the invention as defined in the appended claims.

Claims

Claims1. Decorative element (100) for watchmaking or jewelry, said decorative element (100) being provided with a black optical coating (102) and comprising:- a substrate (1) of monocrystalline silicon;- a multilayer coating (200) provided on said substrate (1), wherein said multilayer coating (200) comprises at least two sequences of sublayers (2, 3) each comprising at least one light-absorbing sublayer (2) and at least one transparent sublayer (3), said multilayer coating (200) being configured so as to give said decorative element (100) a substantially black appearance; characterised in that said substrate (1) has an Ra roughness of between 1 .5 pm and 10 pm, and in that said roughness is formed by a substantially random pyramidal surface texture with a base angle of 54°-55°.

2. Decorative element (100) for watchmaking or jewelry, said decorative element (100) being provided with a black optical coating (102) and comprising:- a substrate (1);- a multilayer coating (200) provided on said substrate (1), wherein said multilayer coating (200) comprises at least two sequences of sublayers (2, 3) each comprising at least one light-absorbing sublayer (2) and at least one transparent sublayer (3), said multilayer coating (200) being configured so as to give said decorative element (100) a substantially black appearance; characterised in that said substrate (1) has an Ra roughness of between 0.02 pm and 4 pm, preferably between 0.05 pm and 1 pm, and in that said roughness is formed by a pyramidal surface texture with a base angle of 35° to 70° preferably 45° to 60°.

3. Decorative element (100) according to any preceding claim, wherein said at least one light-absorbing sublayer (2) has an extinction coefficient above 0.1 at least for wavelengths between 400 nm and 800 nm.

4. Decorative element (100) according to the preceding claim, wherein said at least one light-absorbing sublayer (2) is chosen from a metal, a metal alloy, a lightabsorbing nitride, or a light-absorbing carbide alloy.

5. Decorative element according to the preceding claim, wherein said multilayer coating (200) does not contain chromium, a chromium alloy or a chromium compound.

6. Decorative element (100) according to any preceding claim, wherein said at least one transparent sublayer (3) has a refractive index between 1 .3 and 2.5 at least for wavelengths in the 400 nm to 800 nm range.

7. Decorative element (100) according to the preceding claim, wherein said at least one transparent sublayer (3) is a dielectric layer.

8. Decorative element (100) according to the preceding claim, wherein said at least one transparent sublayer (3) is chosen from transparent oxides, transparent nitrides, transparent oxynitrides, transparent polymers and / or resins.

9. Decorative element (100) according to any preceding claim, wherein said substrate (1 ) is glass, metal, ceramic or silicon.

10. Decorative element (100) according to the preceding claim, wherein said substrate (1) is monocrystalline silicon, and wherein said roughness is formed by a substantially random pyramidal surface texture.

11. Decorative element (100) according to any preceding claim, wherein said multilayer coating (200) has a thickness between 150 nm and 1000 nm, preferably between 200 nm and 600 nm.

12. Decorative element (100) according to any preceding claim, wherein said multilayer coating (200) comprises from 2 to 6 of said sequences, preferably 2 or 3 of said sequences.

13. Decorative element (100) according to any preceding claim, wherein at least some, preferably all, of said least one light-absorbing sublayer (2) and said at least one transparent sublayer (3) have a thickness between 1 nm and 500 nm, preferably between 10 nm and 200 nm.

14. Decorative element (100) according to any preceding claim, wherein said decorative element (100) is chosen from the list containing:- A watch dial - A watch hand- A bezel- An element of a watchcase- A visible element of a piece of jewelry 15. Timepiece comprising a decorative element (100) according to any preceding claim.

16. Piece of jewelry comprising a decorative element (100) according to any of claims

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